1. Field of the Invention
The present invention relates to a melt processable thermoplastic lignin polycondensate and a method for preparing the same.
2. Description of the Related Art
Lignin, together with cellulose and hemicellulose, is one of the principal constituents of wood and is a polymer of phenylpropanoids that exists in a form bound to carbohydrates other than cellulose. Lignin is a complex polymer of three kinds of different phenylpropanoids as constituent units and has a dendritic structure. The constituent units of lignin are represented by Formulae I to III:

Lignin is estimated to have a molecular formula ranging from C18H24O11 to C40H45O18 although its exact chemical structure is not still clearly understood.
The contents of lignin in needleleaf and broadleaf trees are appropriately 25-30% and 20-25%, respectively. Lignin is obtained in a large amount as a by-product from biomass production, kraft pulping, and other processes.
For example, lignin may be extracted from black liquor (see FIG. 1). Black liquor refers to a waste produced during wood delignification in the kraft pulping process. Such black liquor contains lignin residues, hemicelluloses, inorganic compounds, etc. dissolved therein.
Roughly 100 million tons of black liquor as a by-product are produced annually worldwide. The treatment of black liquor incurs a considerable cost and causes environmental pollution. For these reasons, black liquor is difficult and troublesome to treat. The content of lignin in black liquor is estimated to be as high as about 30 million tons per year.
At present, almost all (≧99%) lignin extracted around the world is burned to ashes. Only about 1% of lignin is used as a support material for agrochemicals, a concrete additive, a surfactant, an adsorbent, a fertilizer, a rubber additive, a raw material for carbon particles, etc. Attempts have been made to utilize lignin in other applications. For example, lignin degradation products are used as raw materials for phenol-formaldehyde resins. Lignin is blended with commercially available polymers, such as urea-formaldehyde, epoxy resins, polypropylene, and polystyrene. However, lignin lacks thermal stability and the characteristics of thermoplastic polymers, thus being unsuitable for use as a physical additive. Accordingly, a lignin is not widely applicable but extremely small amount of lignin is utilized at laboratory level.
As can be seen in FIG. 1, lignin is obtained as a reddish brown extract from black liquor. The natural lignin extract is a thermosetting polymer and is maintained in the form of a solid due to its lack of thermoplastic properties, thus being impossible to melt process.
When the lignin extract is heated, it is converted into a black powder (see FIG. 2). That is, the lignin extract evolves odor and fumes and still remains as a powder without being melted when heated. When approaching 100° C., the lignin extract produces odor and fumes due to the presence of low molecular weight molecules. The natural lignin extract is not compatible with general polymers because of its hydrophilic nature, which makes it difficult to obtain excellent physical properties of the lignin extract even when blended with the general polymers.
The reason why it is technically difficult to obtain thermoplastic lignin materials is because lignin has two or more hydroxyl reactive groups. When two or more reactive groups participate in a reaction, three-dimensional crosslinking occurs unavoidably, and as a result, a thermosetting material is produced. As already mentioned, the crosslinked material is impossible to melt process. Lignin obtained from wood extracts by the kraft pulping process is a crosslinked thermosetting material that is not applicable to melt process.
A thermoplastic material refers to a polymer or condensation product that undergoes a phase change to a liquid when heated. A great deal of research is aimed at the development of thermoplastic lignin whose usefulness is expected to expand.
For example, Korean Patent Publication No. 1991-0018395 suggests a method for preparing thermoplastic lignin, including extracting a steam-exploded lignocellulosic material, separating a portion remaining insoluble in water from the lignocellulosic material, extracting the water-insoluble portion with an alcohol or caustic solution, collecting lignin from the extract, and treating the lignin with a halocarbon solvent. This patent publication introduces that the thermoplastic lignin has an average molecular weight ranging from 800 to 1,000 daltons and a melting point ranging from about 125° C. to about 135° C. However, this method is associated with the separation/collection of thermoplastic lignin from a natural material. Only a small amount of the thermoplastic lignin is acquired, which is undesirable from an economic viewpoint, and the molecular weight of the thermoplastic lignin is not adjustable. Such physical properties make the thermoplastic lignin unsuitable for commercialization. Another drawback of lignin is poor thermal stability. When heated to 180° C., a normal polymer melt processing temperature, lignin evolves severe fumes and odor accompanied by degradation. Accordingly, the viscosity of lignin is impossible to measure and general processes are impossible to apply to lignin.
Further, International Patent Publication No. WO2012/038259 discloses a method for producing a carbon fiber, including extracting lignin from wood, separating/collecting a thermoplastic portion from the lignin by elution with a solvent, and carbonizing the thermoplastic lignin. Specifically, this patent publication introduces that the thermoplastic lignin has a glass transition temperature of 90 to 160° C., a dispersity of less than 28, an ash content of less than 1% by weight, and a volatile content of less than 1% by weight after high temperature stabilization. However, the lignin is a naturally occurring substance that contains a large amount of a thermosetting portion and a small amount of a thermoplastic portion. Accordingly, this method is disadvantageous from an economic viewpoint. The thermoplastic portion of the natural lignin is water soluble and has a low molecular weight, which are obstacles to its commercialization.